Chapter 18.4 & 21: Fatty acids, Soaps, Lipids

What do the fatty acid components of triglycerides have in common?

1. Practically all are unbranched carboxylic acids. 2. They range in size from about 10 to 20 carbons. 3. They contain an even number of carbon atoms. 4. Apart from the -COOH group, they have no functional groups, except that some have carbon-carbon double bonds. 5. In most fatty acids that have carbon-carbon double bonds, the cis isomers predominate.

Lipids

A heterogeneous class of naturally occurring organic compounds classified together on the basis of common solubility properties. • Insoluble in water, but soluble in organic solvents including diethyl ether, dichloromethane, and acetone.

Triglyceride

A triester of glycerol with three fatty acids. • In most triglycerides, two or three different fatty acid components are present. • The hydrophobic character is caused by the long hydrocarbon chains. • The ester groups, although polar, are buried within a nonpolar environment, which makes triglycerides insoluble in water.

State of unsaturated fatty acids at room temperature

All unsaturated fatty acids are liquids at room temperature because the cis double bonds interrupt the regular packing of their hydrocarbon chains.

Biological membranes

Complex lipids form the membranes around cells and around small structures within cells.

How do biological membranes behave in aqueous solutions?

In aqueous solution, complex lipids spontaneously form into a lipid bilayer, with a back-to-back arrangement of lipid monolayers. • Polar (hydrophilic) head groups are in contact with the aqueous environment. • Nonpolar (hydrophobic) tails are buried within the bilayer and shielded from the aqueous environment. • The major force driving the formation of lipid bilayers is hydrophobic interaction. • The arrangement of hydrocarbon tails in the interior can be rigid (if rich in saturated fatty acids) or fluid (if rich in unsaturated fatty acids).

How do soaps behave in water?

In water, soap molecules spontaneously cluster into micelles, a spherical arrangement of molecules such that their hydrophobic parts are shielded from the aqueous environment, and their hydrophilic parts are in contact with the aqueous environment.

Fatty acid

Long chain carboxylic acids derived from animal fats, vegetable oils, or phospholipids of biological membranes. • More than 500 have been isolated from various cells and tissues. • Most have between 12 and 20 carbons in an unbranched chain. • In most unsaturated fatty acids, the cis isomer predominates; trans isomers are rare.

Physical properties of triglycerides

The physical properties of triglycerides depend on the fatty acid components. • Melting points of fatty acids increase as the number of carbons in the hydrocarbon chains increases and as the number of double bonds decreases. • Triglycerides rich in unsaturated fatty acids are generally liquid at room temperature and are called oils • Triglycerides rich in saturated fatty acids are generally semisolids or solids at room temperature and are called fats.

Hardening of triglycerides

The reduction of some or all of the carbon-carbon double bonds of an unsaturated triglyceride using H2/transition metal catalyst, which converts a liquid triglyceride to a semisolid. • In practice, the degree of hardening is carefully controlled to produce fats of a desired consistency. • The resulting fats are sold for kitchen use (Crisco, Spry, Dexo, and others). • Margarine and other butter substitutes are produced by partial hydrogenation of polyunsaturated oils derived from corn, cottonseed, peanut, and soybean oils. • The hardening process is the source of trans fatty acids.

State of saturated fatty acids at room temperature

The regular nature of their hydrocarbon chains allows them to pack together in such a way as to maximize interactions (by London dispersion forces) between their chains.

Unsaturated fatty acid melting points

Unsaturated fatty acids have lower melting points than their saturated counterparts. The greater the degree of unsaturation, the lower the melting point. Compare, for example, the melting points of the following 18-carbon fatty acids: Linolenic acid, with three carbon-carbon double bonds, has the lowest melting point as compared to stearic acid, oleic acid, and linoleic acid.

How do soap micelles behave in water when combined with dirt or grease?

When soaps and dirt, such as grease, oil, and fat stains are mixed in water, the nonpolar hydrocarbon inner parts of the soap micelles "dissolve" the nonpolar substances.

Groups that are part of the lipids category

• Fatty acids, triglycerides, sphingolipids, phosphoacylglycerols, and glycolipids. •Lipid-soluble vitamins. •Prostaglandins, leukotrienes, and thromboxanes. • Cholesterol, steroid hormones, and bile acids.

Soaps

• Natural soaps are sodium or potassium salts of fatty acids. • They are prepared from a blend of tallow and palm oils (triglycerides). • Triglycerides are triesters of glycerol. • The solid fats are melted with steam and the water insoluble triglyceride layer that forms on the top is removed.